In the industrial preparation of dimethyl carbonate (DMC), there is frequently formation of mixtures of DMC and methanol which may contain further constituents. Thus, for example, EP 413215 describes a process for preparing DMC in which DMC is formed in admixture with methanol and water.
However, to use DMC as reagent for further reactions, such as the transesterification of other carbonates, in particular to give diphenyl carbonate or other diaryl carbonates, but also aryl alkyl, dialkyl and dialkenyl carbonates, it is necessary to be able to use it in essentially pure form, in particular essentially free of methanol. For this purpose, an effective process for very complete separation of other materials, in particular of methanol, from mixtures of DMC with such other materials is of great importance.
In numerous uses of DMC, for example for carrying out transesterification reactions, in particular for the preparation of monoaryl and diaryl carbonates, it is not possible to make the DMC react completely. Such reactions give, besides the desired reaction products, mixtures which, in addition to the methanol liberated, sometimes also contain appreciable amounts of the unreacted useful material DMC.
Separation of methanol from such mixtures in very pure form allows, on the one hand, its renewed provision for an economic use, e.g. for the purpose of recycling to the preparation process for DMC. On the other hand, the residual amounts of DMC present in such mixtures are recovered in usably enriched form, which is a further economic advantage.
It can therefore be said in summary that, in particular for the purpose of possible use of DMC as substitute material for the highly toxic phosgene, there has to be available not only an economically satisfactory preparation process, but also an efficiently operating separation process for the abovementioned mixtures of methanol and DMC and possibly further components.
The mixture specified can be separated by distillation only with great difficulty, i.e. not in a simple manner, since DMC forms an azeotrope with methanol at atmospheric pressure, this azeotrope having the approximate composition 70% of methanol, 30% of DMC.
Separation of such mixtures by exploitation of the pressure dependence of the composition of the azeotrope, for instance by distillation at higher or lower pressures than atmospheric pressure or by successive distillation at higher and lower pressures than atmospheric pressure (two-pressure distillation), as is described, for example, in JP 02212456 (cited according to CA 114, 81029), is associated with high energy and capital costs.
To carry out a distillative separation of mixtures of methanol and DMC, it has also been proposed that the distillation be carried out with the addition of further auxiliaries. Thus, according to German Offenlegungsschrift 2737265, the addition of hydrocarbons such as n-heptane is supposed to make possible a separation of the two components by means of azeotropic distillation; according to German Offenlegungsschrift 2706684, the distillative separation of mixtures of DMC and methanol can be carried out by means of an extractive distillation with the addition of organic solvents such as methyl-glycol acetate. In both variants, it is necessary to separate off the auxiliary and circulate it. In addition, the purity of the dimethyl carbonate can be impaired by the presence of further materials.
Besides these thermal processes, other processes for separating off methanol from mixtures with DMC have also been proposed. Thus, for example, U.S. Pat. No. 4,960,519 describes pervaporation on specific membranes. However, this separation technique generally has to be carried out in association with additional distillative separation operations and, owing to the complicated apparatus used, is associated with high costs which make industrial utilization seem questionable.
Furthermore, U.S. Pat. No. 4,582,645 proposes removing DMC from mixtures by absorption on zeolites. Owing to the limited absorption capacity of the zeolites (100 mg of DMC/2.5 g of zeolite), only small amounts of DMC can be separated off in this way. This process is therefore only suitable for mixtures whose DMC content is small.
It is accordingly an object of the invention to find a process which makes it possible to separate off methanol from mixtures with DMC in an economically useful manner.